This invention relates generally to the formation of phosphate conversion coatings on metals and specifically to a process for monitoring the amount of grain refiner in the coating bath.
Baths for the formation of phosphate conversion coatings are used to form coatings on metals such as steel, iron, zinc, galvanized steel, cadmium and aluminum. The coatings provide adhesion promotion and corrosion resistance to the metal surfaces when the surfaces are painted. One type of coating bath contains divalent metal salts that form insoluble phosphates on the metal surface. The metal ions commonly present in these salts include zinc, zinc-nickel, zinc-magnesium, zinc-calcium, zinc-manganese, and manganese. The baths can be improved by the inclusion of grain refiners which act to make the coatings microcrystalline. The grain refiner provides thinner coatings having improved adhesion and enables metal sheets to be painted prior to forming operations which would otherwise cause paint cracking. Such coating baths with grain refiners are described, for example, in copending application Ser. No. 342,279, filed Jan. 25, 1982. The grain refiners described in this application are certain acidic organic phosphates and phosphonates. Effective amounts of certain of these grain refiners are in the range of only about 25-200 parts per million. For commercial operations, it is necessary to be able to monitor these very small amounts of grain refiner. One method which is frequently used to quantitatively determine phosphonates is to oxidize them to phosphates and analyze for the amount of phosphate present. With inorganic phosphate containing coating baths, this method, of course, cannot be used because of the large amounts of phosphate already present in the bath. Any method, to be successful, must also be relatively insensitive to the presence of contaminants which build up in the coating baths during use. I have now provided a method which permits the quantitive determination of small amounts of phosphorous containing grain refiner in a phosphate conversion coating bath.